During milk separation, particularly during separation of cold milk (normally at about 2 to 5° C.), the cream separated from the milk may cause problems in the form of obstruction of various flow passages in the separator, and the narrow flow gaps between the separation discs are particularly problematic. Cold cream is of relatively high viscosity and obstructs the flow gaps by adhering to the surfaces of the separation discs and forming accumulations in the flow gaps. Cold cream also has non-Newtonian characteristics such that the viscosity depends on the flow velocity. This means that cold cream has a so-called liquid limit at which its movement ceases completely (the cream stagnates) when the flow velocity or the shear stresses acting upon the cream is/are below a certain value. To remedy this obstruction problem, the cold milk may be heated (or at least not cooled) before separation. In addition to the advantage of the cream not tending to stagnate, it is also relatively easier to separate cream from non-cold milk, making it possible to separate a larger amount of non-cold milk per unit time. However, heating the milk involves extra equipment and access to thermal energy. Heating the milk may also adversely affect its taste.
SE115167 describes a known separator, wherein the separation disc is given a shape without entailing any disadvantage in the form of, for example, stagnation of separated light phase. The separator has a column in the form of a central tube connected to the rotor. The separation disc forms together with other separation discs a stack of separation discs supported by protrusions in the form of wings on the central tube. The separation disc is provided with a hole which surrounds the axis of rotation and which is adapted to forming, in the stack of separation discs, a flow passage which extends axially through the stack of separation discs to lead away light phase separated from the liquid. The inner edge of the flange element thus delimits together with the outer circumference and wings of the central tube an axial throughflow cross-section of the axial flow passage for the light phase. The velocity of the light phase in the axial flow passage within the inner edge of the separation discs depends on the throughflow cross-section of the axial flow passage and the flow through the separator (in other words, the amount of liquid per unit time which is caused to flow through the whole separator). As the cream takes longer to separate from cold milk than from non-cold milk, a relatively smaller flow through the separator is normally required during separation of cold milk as compared with non-cold milk. With the same throughflow cross-section for the light phase, a smaller flow through the separator will result in a lower flow velocity through the flow passage for the light phase. However, a lower flow velocity entails increased risk of stagnation of the cold cream in the flow passage. The flow velocity of the cold cream may be raised by reducing the throughflow cross-section. In the case of the known separation disc, the throughflow cross-section depends on the diameter of the separation disc's hole which surrounds the axis of rotation in combination with the dimensions of the central tube and of the wings disposed thereon. The known separation disc thus has the disadvantage that the throughflow cross-section depends on said combination. A change of velocity can therefore only be achieved if the whole subassembly (central tube with separation discs) is replaced, since the diameter of the holes in the separation discs and the size of the wings on the central tube are mutually dependent.
GB191404028 describes a known separator for separation of cold milk. The separators rotor comprises a number of stacked separation discs adapted to being rotatable with the rotor, wherein adjacent separation discs form between them narrow flow gaps for separation of cold cream from cold milk fed in. The separation disc is provided with a hole which surrounds the axis of rotation and which is adapted to forming in the stack of separation discs a flow passage which extends axially through the stack of separation discs. Each separation disc is provided with a number of holes for the cream to flow through which are distributed on a radius close to the hole which surrounds the axis of rotation. The object of these throughflow holes is for said obstruction to be reduced by the separation disc having a relatively smaller surface to which the cream can adhere than in the case of a separation disc without throughflow holes. However, the throughflow holes also result in reduced strength and rigidity of the separation disc relative to a separation disc without such throughflow holes. During operation of the separator, the centrifugal force generated by rotation will cause stresses on the separation disc. Such stresses may not only deform the separation disc but also cause fatigue damage in the form of cracking in the separation disc, which often occurs at holes in the separation disc, e.g. at said throughflow holes.